Generally, polyimide (PI) resin refers to highly heat-resistant resin obtained by ring closure and dehydration of polyamic acid at high temperature, which is obtained by solution polymerization of aromatic dianhydride and aromatic diamine or aromatic diisocyanate. For the preparation of the polyimide resin, the aromatic dianhydride includes, for example, pyromellitic dianhydride (PMDA) or biphenyl tetracarboxylic dianhydride (BPDA), and the aromatic diamine includes, for example, oxydianiline (ODA), p-phenylene diamine (p-PDA), m-phenylene diamine (m-PDA), methylene dianiline (MDA), and bisaminophenylhexafluoropropane (HFDA).
Since polyimide resin, which is insoluble, infusible and super high heat resistant, has superior properties, including heat and oxidation resistance, radiation resistance, cryogenic resistance properties, and chemical resistance, it has been used in various fields, including advanced heat resistant materials, such as automobile materials, aircraft materials, or spacecraft materials, and electronic materials, such as insulation coating agents, insulating films, semiconductors, or electrode protective films of TFT-LCDs. Recently, polyimide resin has been used as display materials, such as optical fibers or liquid crystal alignment layers, and transparent electrode films, which are constructed by mixing conductive fillers with polymers or applying conductive fillers to the surface of polymer films.
However, a high aromatic ring density and a charge transfer interaction of polyimide resin cause it to be colored brown or yellow, undesirably resulting in low transmittance in the visible light range. Such yellow or brown color of polyimide resin makes it difficult to apply it to the fields requiring transparency.
In order to solve such problems, attempts to realize methods of purifying a monomer and a highly pure solvent in order to be polymerized have been made, but the improvement in transmittance was not large.
U.S. Pat. No. 5,053,480 discloses a method of using an alicyclic dianhydride component instead of the aromatic dianhydride. Although this method improves transparency and color in a solution phase or a film phase compared to the purification methods, the improvement in transmittance is limited, and therefore high transmittance is not realized, and also, the thermal and mechanical properties thereof are deteriorated.
In U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036, and 6,232,428, and Korean Unexamined Patent Publication No. 2003-0009437, there have been reports related to the preparation of polyimide, having a novel structure, which is improved in terms of transmittance and color transparency within a range in which the thermal properties are not greatly decreased, using aromatic dianhydride and aromatic diamine monomers, having a linker, such as —O—, —SO2—, or CH2—, a bent structure due to connection not at the p-position but at the m-position, or a substituent, such as —CF3. However, such a polyimide can be confirmed to have mechanical properties, a yellow index, and visible light transmittance insufficient for use in semiconductor insulating films, TFT-LCD insulating films, electrode protective films, and flexible display substrates.